Foamed polyolefin resin beads

ABSTRACT

The present invention relates to foamed polyolefin resin beads. Further, the present invention provides foamed resin beads obtained by foaming and expanding composite resin beads which include a core layer constituted by a polyolefin resin and a covering layer which covers the core layer constituted by a polyolefin resin, wherein (a) the polyolefin resin constituting the core layer is a crystalline polyolefin resin, (b) the polyolefin resin constituting the covering layer is a crystalline polyolefin resin which has a lower melting point (B) than a melting point (A) of the polyolefin resin constituting the core layer, wherein a temperature difference [(A)−(B)] between the melting point (B) and the melting point (A) is more than 0° C. and 80° C. or less, or a noncrystalline polyolefin resin which has a softening point (C) lower than the melting point (A) of the polyolefin resin constituting the core layer, wherein a temperature difference [(A)−(C)] between the softening point (C) and the melting point (A) is more than 0° C. and 100° C. or less, and 10% by weight or more and less than 50% by weight of polymer type antistatic agent is contained in the covering layer. The foamed polyolefin resin beads of the present invention provide foamed polyolefin resin beads are excellent in fusion properties between beads at the time of molding in a mold, capable of providing a molded foamed article which is excellent antistatic performance, has no deterioration of the antistatic performance with age, whose antistatic performance is not humidity dependent, does not contaminate packaging products, has a good molded foamed article surface, and has excellent mechanical properties.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to foamed polyolefin resin beads which areexcellent in fusion properties between beads at the time of molding in amold, capable of providing a molded foamed article which is excellentantistatic performance, has no deterioration of the antistaticperformance with elapse of the time, whose antistatic performance is nothumidity dependent, does not contaminate packaging products, has a goodmolded foamed article surface, and has excellent mechanical properties.The present invention provides foamed polyolefin resin beads and amolded article obtained by molding the beads in a mold which are used asshock absorbing materials, heat insulating materials, or packagingmaterials in the electronic or automotive industry or useful forapplication to returnable boxes.

2. Description of the Related Art

Foamed polyolefin resin beads and a molded foamed article produced bymolding the foamed polyolefin resin beads in a mold have been widelyused for packaging of electronic devices such as OA instruments andparts or precision instruments and parts and further used as cushioningpackaging materials. Polyolefin resin is easily charged, which causesadhesion of dust and damage to equipment parts. Therefore, a moldedfoamed article composed of foamed polyolefin resin beads to whichantistatic performance is given has been used.

Examples of the method for imparting the antistatic performance to thefoamed polyolefin resin beads include a method for applying a surfactantto the surface of the molded article and a method for molding foamedbeads in which a polyolefin resin into which the surfactant is kneadedis foamed. A molded article composed of foamed beads in which thepolyolefin resin into which the surfactant is kneaded has been used as asuitable material with antistatic performance. However, when theantistatic performance is achieved by the surfactant, the surfactantpresent on the surface of the molded article surface adsorbs moisture inthe air. Therefore, it is difficult to produce the antistatic effectunder an environment of low humidity, particularly in winter.Additionally, there have been problems of contamination due to theadhesion of the surfactant to packaging products and decrease inperformance with age.

In the method for applying the surfactant to the surface of the moldedarticle, the surfactant is easily flaked off from the surface of themolded article and the antistatic effect cannot be produced after theseparation.

As a method for solving the problems of the adhesion of the surfactantto packaging products, contamination of packaging products, and flakingof the surfactant when the surfactant are used, foamed polyolefin resinbeads containing a hydrophilic polymer and the surfactant and the moldedfoamed article obtained by molding the foamed polyolefin resin beads ina mold are disclosed in Japanese Patent Application Laid-Open (JP-A) No.2000-290421. The invention described in JP-A No. 2000-290421 is that thesurfactant is trapped between molecules of the hydrophilic polymer,thereby preventing the surfactant from transferring to packagingproducts.

Alternatively, foamed resin beads are produced by foaming and expandingresin particles to which polymer type antistatic agent containing thehydrophilic polymer as an antistatic agent is added, thereby preventingthe surfactant from transferring to packaging products and antistaticperformance from being reduced, which is well known.

A technique which gives antistatic performance by coating the surface offoamed polyolefin resin beads with an adhesive resin containing thepolymer type antistatic agent is disclosed in JP-A No. 2002-3634.

Japanese Patent Application Publication (JP-B) No. 62-34336 and JP-A No.10-77359 are disclosed foaming and expanding foamed resin beads composedof a core layer including the polyolefin resin and a covering layerprepared from the polyolefin resin having a melting point lower thanthat of the polyolefin resin in the core layer in order to improvefusion bonding properties between the foamed resin beads.

Although a method described in JP-A No. 2000-290421 prevents thesurfactant from transferring (transcription) to packaging products,fusion bonding properties of foamed resin beads are reduced when thehydrophilic polymer is added to the foamed resin beads. Further, sincethe hydrophilic polymer has water absorption properties, when resinparticles are heated in an aqueous medium under pressure, resinparticles absorb water. When the expansion ratio is high, the pressurein the cells of the obtained foamed resin beads is reduced by watercondensation, which causes the phenomenon of shrinkage of beads inthemselves.

In order to achieve a high antistatic performance by the polymer typeantistatic agent, it is necessary to add a large amount of theantistatic agent to the foamed resin beads. However, when the additiveamount of the polymer type antistatic agent containing the hydrophilicpolymer (hereinafter simply referred to as “polymer type antistaticagent” or “antistatic agent”) is increased to obtain a high antistaticperformance, the foaming properties of foamed resin beads are inhibitedand the fusion bonding properties between the foamed resin beads arereduced with the increase in the additive amount. When the heatingtemperature at the time of molding is increased to improve the fusionbonding properties between the foamed resin beads, the foamed resinbeads cannot resist the heating temperature. As a result, the foamedresin beads are shrunk or fused to not only the surface of them but alsothe inside of them, thereby causing serious damage to the cell structureof the obtained molded foamed article. On the other hand, when theheating temperature is lowered, fusion bonding properties of foamedresin beads are reduced and the surface condition of the molded foamedarticle is significantly deteriorated. Thus, a good molded foamedarticle cannot be obtained. Particularly, the phenomenon issignificantly observed in foamed resin beads having a high expansionratio.

A method described in JP-A No. 2002-3634 involves a process of kneadingthe polymer type antistatic agent with the adhesive resin and coatingthe surface of foamed resin beads with the adhesive resin containing thepolymer type antistatic agent and thus the production step iscomplicated. When the adhesive resin is used, foamed resin beads areeasily adhered each other in the coating step. Further, the chargingproperty of the foamed resin beads to a molding cavity may be unstable.

SUMMARY OF THE INVENTION

According to the present invention, there is provided the foamedpolyolefin resin beads, in which shrinkage of the foamed resin beadswhich is observed when the polymer type antistatic agent is mixed withthe foamed polyolefin resin beads is suppressed and fusion bongingproperties between foamed resin beads are good.

Further, according to the present invention, there is provided themolded foamed article of polyolefin resin which is molded in a moldusing the foamed polyolefin resin beads and has the antistaticperformance, a good surface condition, and an excellent mechanicalstrength.

In the present invention, it is found that composite resin beads includea core layer constituted by a crystalline polyolefin resin, a coveringlayer constituted by a crystalline polyolefin resin having a meltingpoint lower than that of the resin constituting the core layer or acovering layer constituted by a noncrystalline polyolefin resin having asoftening point lower than the melting point of the resin constitutingthe core layer and the polymer type antistatic agent is blended with thecovering layer, which is foamed and expanded and the obtained foamedpolyolefin resin beads can achieve the above-described object.

That is, the present invention provides the following:

[1] foamed polyolefin resin beads obtained by foaming and expandingcomposite resin beads which include a core layer constituted by apolyolefin resin and a covering layer which covers the core layerconstituted by a polyolefin resin, wherein

(a) the polyolefin resin constituting the core layer is a crystallinepolyolefin resin,

(b) the polyolefin resin constituting the covering layer is acrystalline polyolefin resin which has a lower melting point (B) than amelting point (A) of the polyolefin resin constituting the core layer,wherein a temperature difference [(A)−(B)] between the melting point (B)and the melting point (A) is more than 0° C. and 80° C. or less, or anoncrystalline polyolefin resin which has a softening point (C) lowerthan the melting point (A) of the polyolefin resin constituting the corelayer, wherein a temperature difference [(A)−(C)] between the softeningpoint (C) and the melting point (A) is more than 0° C. and 100° C. orless, and 10% by weight or more and less than 50% by weight of polymertype antistatic agent is contained in the covering layer.

[2] the foamed polyolefin resin beads according to a first aspect,wherein the polyolefin resin constituting the covering layer is acrystalline polyolefin resin which has a lower melting point (B) than amelting point (A) of the polyolefin resin constituting the core layer,wherein a temperature difference [(A)−(B)] between the melting point (A)and the melting point (B) is in the range of 1 to 80° C., or anoncrystalline polyolefin resin which has a lower softening point (C)than a melting point (A) of the polyolefin resin constituting the corelayer, wherein a temperature difference [(A)−(C)] between the meltingpoint (A) and the softening point (C) is in the range of 1 to 100° C.;

[3] the foamed polyolefin resin beads according to the first aspect,wherein the polyolefin resin constituting the covering layer is acrystalline polyolefin resin which has a lower melting point (B) than amelting point (A) of the polyolefin resin constituting the core layer,wherein a temperature difference [(A)−(B)] between the melting point (A)and the melting point (B) is in the range of 5 to 60° C., or anoncrystalline polyolefin resin which has a lower softening point (C)than a melting point (A) of the polyolefin resin constituting the corelayer, wherein a temperature difference [(A)−(C)] between the meltingpoint (A) and the softening point (C) is in the range of 5 to 60° C.;

[4] the foamed polyolefin resin beads according to the first aspect,wherein the core layer does not substantially contain the polymer typeantistatic agent.

[5] the foamed polyolefin resin beads according to the first aspect,wherein 5 to 15% by weight of polymer type antistatic agent is containedin the core layer.

[6] the foamed polyolefin resin beads according to the first aspect,wherein the crystalline polyolefin resin constituting the core layer isa polypropylene resin.

[7] the foamed polyolefin resin beads according to the sixth aspect,wherein the polyolefin resin constituting the covering layer is thepolypropylene resin.

[8] the foamed polyolefin resin beads according to the first aspect,wherein a weight ratio of the core layer and the covering layer is inthe range of 99.5:0.5 to 80:20.

[9] the foamed polyolefin resin beads according to the first aspect,wherein the weight ratio of the core layer and the covering layer is inthe range of 98:2 to 80:20.

[10] the foamed polyolefin resin beads according to the first aspect,wherein the weight ratio of the core layer and the covering layer is inthe range of 96:4 to 90:10.

[11] the foamed polyolefin resin beads according to the first aspect,wherein the polyolefin resin constituting the covering layer is apolyolefin resin polymerized with a metallocene polymerization catalyst.

[12] the foamed polyolefin resin beads according to the first aspect,wherein the covering layer of the foamed beads is substantially solid.

[13] the foamed polyolefin resin beads according to the first aspect,wherein a ratio (X/Y) of an apparent density (X) of foamed beads afterpressurizing with a compressed air under conditions of 30° C. and 0.2MPa (G) for 24 hours and leaving under an ordinary pressure at 23° C.for 24 hours to an apparent density (Y) of foamed beads before thepressurization is in the range of 0.8 to 1.0.

Further, the present invention relate to a molded foamed article ofpolyolefin resin produced by molding the foamed polyolefin resin beadsin a molding cavity according to the first to thirteenth aspects, wherethe molded foamed article has the surface resistivity of less than1×10¹⁴Ω.

The foamed polyolefin resin beads (hereinafter may be referred to assimply “foamed resin beads” or “foamed beads”) of the present inventionare foamed resin beads obtained by foaming and expanding composite resinbeads including the core layer and the covering layer, the resinconstituting the core layer is the crystalline polyolefin resin, thecovering layer includes the crystalline polyolefin resin having themelting point lower than that of the crystalline polyolefin resinconstituting the core layer or the noncrystalline polyolefin resinhaving the softening point lower than the melting point of thecrystalline polyolefin resin constituting the core layer, and a specificamount of the polymer type antistatic agent is blended with the coveringlayer. Therefore, there is no shrinkage of foamed resin beads or theshrinkage is small due to the water absorption of the antistatic agent.Further, molding in a mold can be performed without causing damage tothe cell structure of the core layer of the foamed resin beads whenfoamed resin beads are heated at a fusable temperature and foamed resinbeads are excellent in fusion bonding properties.

The foamed polyolefin resin beads in the present invention can providethe molded foamed article which is excellent in antistatic performance,prevents or reduces the transfer (transcription) of the antistatic agentto packaging products, hardly has deterioration of the antistaticperformance with age, whose antistatic performance is hardly humiditydependent, whose shrinkage after molding is sufficiently suppressed, andhas a good surface condition, and an excellent mechanical strength.

The foamed resin beads of the present invention have the multilayeredstructure as described above. When the antistatic agent is blended withthe covering layer at a predetermined ratio, the desired antistaticeffect can be obtained. Thus, it is not necessarily needed that theantistatic agent is blended with the core layer. Generally, a relativelylarge amount of the polymer type antistatic agent is necessary to obtainantistatic effects. In the present invention, the desired antistaticperformance can be exhibited by having the above-described structureeven if the polymer type antistatic agent is not blended with the corelayer or the blending amount of the polymer type antistatic agent issmall. Therefore, foaming properties of the foamed resin beads are notinhibited and are not greatly shrunk, thereby producing the moldedfoamed article having the desired mechanical strength. Further, when theblending amount of the antistatic agent is small based on all of thefoamed resin beads, sufficient antistatic performance can be exhibited.A step of coating foamed resin beads with the resin containing theantistatic agent after production of the foamed resin beads is notnecessary. Thus, foamed resin beads having antistatic properties can beproduced at low cost.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example of a DSC curve in the first measurement offoamed resin beads by heat flux differential scanning calorimetry.

FIG. 2 illustrates an example of a DSC curve in the second measurementof foamed resin beads by heat flux differential scanning calorimetry.Herein, a is an intrinsic peak, b is a high temperature peak, α is apoint corresponding to 80° C. on the DSC curve, β is a pointcorresponding to a melting end temperature, γ is a point correspondingto a valley portion between a and b, σ is a point that crosses a linesegment (α-β), and T is the melting end temperature.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The foamed polyolefin resin beads of the present invention are foamedresin beads obtained by foaming and expanding composite resin beadswhich include a core layer constituted by a polyolefin resin and acovering layer which covers the core layer constituted by anotherpolyolefin resin different from the polyolefin resin of the core layer,where the polyolefin resin constituting the core layer of the compositeresin beads is a crystalline polyolefin resin, the polyolefin resinconstituting the covering layer is a crystalline polyolefin resin whichhas a lower melting point (B) than a melting point (A) of the polyolefinresin constituting the core layer, wherein a temperature difference[(A)−(B)] between the melting point (B) and the melting point (A) ismore than 0° C. and 80° C. or less, or a noncrystalline polyolefin resinwhich has a softening point (C) lower than the melting point (A) of thepolyolefin resin constituting the core layer, wherein a temperaturedifference [(A)−(C)] between the softening point (C) and the meltingpoint (A) is more than 0° C. and 100° C. or less, and 10% by weight ormore and less than 50% by weight of polymer type antistatic agent iscontained in the covering layer.

The foamed polyolefin resin beads of the present invention are foamedresin beads obtained by foaming and expanding composite resin beadsincluding the core layer and the covering layer, and have a compositestructure including the core layer in a foaming state and the coveringlayer which is substantially non-foaming. Hereinafter, the core layerand the covering layer of composite resin beads are referred to as a“core layer (R)” and a “covering layer (R)”, respectively. The corelayer and the covering layer of foamed resin beads obtained by foamingand expanding the composite resin beads are referred to as a “core layer(E)” and a “covering layer (E)”, respectively.

The foamed polyolefin resin beads of the present invention are producedby foaming and expanding the composite resin beads including the corelayer (R) and the covering layer (R). For that reason, in the foamedresin beads, the core layer (R) corresponds to the core layer (E) andthe covering layer (R) corresponds to the covering layer (E). Therefore,the polyolefin resin constituting the covering layer (E) of the foamedresin beads is identical to the polyolefin resin constituting thecovering layer (R). The polyolefin resin constituting the core layer (E)of the foamed resin beads is identical to the polyolefin resinconstituting the core layer (R).

It is preferable that the covering layer (E) of foamed resin beads inthe present invention is a resin layer which is substantially solid.When the covering layer (E) of foamed resin beads is solid, themechanical strength of a molded foamed article which is produced bymolding the foamed resin beads in a mold can be maintained at a highlevel. Here, the term “solid” includes not only a solid in which cellsare not present (including a solid in which cells once formed at thetime of producing the foamed resin beads had been melt-destroyed and thecells were disappeared) but also a solid in which very minute cells areslightly present.

Usually, as the additive amount of the polymer type antistatic agent tothe resin constituting foamed resin beads is increased, fusion bondingproperties between the foamed resin beads are reduced. In order tosufficiently fuse the foamed resin beads, they need to be heated athigher temperatures. In the present invention, the foamed resin beadsare composed of a core layer (E) and covering layer (E), a resinconstituting the covering layer (E) has a melting point or a softeningpoint lower than a melting point of the resin constituting the corelayer (E). Therefore, foamed resin beads of the present invention can befused-bonded at a temperature lower than that of foamed resin beadsincluding a polymeric antistatic agent which do not have the coveringlayer (E) even when a lot of polymer type antistatic agents are added tothe covering layer (E) or foamed resin beads which do not have a meltingpoint difference between the core layer (E) and the covering layer (E)substantially even if they have the covering layer (E). Thus, the foamedresin beads can be fused-bonded without affecting a cell structure ofthe core layer (E) and a high antistatic performance can be achieved.Further, the foamed resin beads of the present invention are produced byfoaming and expanding the composite resin beads in which the antistaticagent is contained in the covering layer (R). Therefore, the antistaticeffect thereof is higher than that of foamed resin beads whose surfaceis simply coated with the resin containing the antistatic agent in thepost process, because of the covering layer (R) is drawn out at the timeof foaming and expanding, the antistatic agent is suitably oriented inthe resin, a network structure of the antistatic agent is constructed.

The term “polyolefin resin” as used herein means a resin correspondingto any of the following (a) to (e):

(a) a homopolymer of ethylene and α-olefins such as propylene andbutene-1 (hereinafter ethylene and α-olefins are collectively referredto as “olefin”.);

(b) a copolymer selected from two or more olefins;

(c) a copolymer of olefin component and other monomer components such asstyrene where an olefin component unit of the copolymer is 30% by weightor more, preferably 50% by weight or more, more preferably 70% by weightor more, further preferably 80% by weight or more, most preferably 90%by weight or more;

(d) two or more mixtures selected from the group consisting of (a), (b),and (c) described above; and

(e) a mixed resin composition of one or two or more selected from thegroup consisting of (a), (b), (c), and (d) described above with otherthermoplastic resins except (a), (b), (c), and (d) described aboveand/or other elastomers where the content of olefin resin in thecomposition is 30% by weight or more, preferably 50% by weight or more,more preferably 70% by weight or more, further preferably 80% by weightor more, most preferably 90% by weight or more.

A preferable example of the polyolefin resin in the present invention isa resin corresponding to any of the following (f) to (i):

(f) propylene homopolymer;

(g) a copolymer of propylene and other monomers where the content ofpropylene is 30% by weight or more, preferably 50% by weight or more,more preferably 70% by weight or more, further preferably 80% by weightor more, particularly preferably 90% by weight or more;

(h) two or more mixtures selected from the group consisting of (f) and(g) described above; and

(i) a resin composition including a mixture of one or two or moreselected from the group consisting of (f), (g), and (h) described abovewith other thermoplastic resins except (f), (g), and (h) described aboveand/or other elastomers where the content of polypropylene resin in themixed resin composition is 30% by weight or more, preferably 50% byweight or more, more preferably 70% by weight or more, furtherpreferably 80% by weight or more, particularly preferably 90% by weightor more.

Specific examples of (a) include polyethylene resin, polypropyleneresin, and polybutene resin. Specific examples of (b) include anethylene-propylene copolymer and an ethylene-propylene-butene-1copolymer. These copolymers may be either a block copolymer or a randomcopolymer.

With reference to (e) or (i) described above, other examples of thethermoplastic synthetic resin and elastomers include vinyl acetateresin, thermoplastic polyester resin, acrylic ester resin, methacrylicester resin, polystyrene resin, polyamide resin, fluorocarbon resin,ethylene-propylene rubber, ethylene-propylene-diene rubber,ethylene-acrylic rubber, chlorinated polyethylene rubber, andchlorosulfonated polyethylene rubber.

The polyolefin resin constituting the core layer (R) of the presentinvention is the crystalline polyolefin resin. The term “crystallinepolyolefin resin” used herein means a polyolefin resin which shows aclear endothermic peak accompanying the melting of the polyolefin resinof a DSC curve when a DSC curve is taken at a heating rate of 10° C./minwith a heat flux differential scanning calorimetry apparatus(hereinafter referred to as a DSC apparatus) in accordance with “when amelting temperature is measured after performing a constant heattreatment” described in JIS K7121 (1987) (both a heating rate and acooling rate in controlling the condition of the specimen are 10° C./min). In this regard, a lower limit of the calorific value of endothermicpeak is 2 J/g. On the other hand, the polyolefin resin which does notexhibit a clear endothermic peak, that is, whose calorific value of theendothermic peak is less than 2 J/g is a noncrystalline polyolefinresin.

Examples of the crystalline polyolefin resin constituting the core layer(R) of the present invention include polyethylene resin, polypropyleneresin, and polybutene resin. Propylene homopolymer, ethylene-propylenecopolymer, and ethylene-propylene-butene-1 copolymer are preferablesince they are excellent in balance between heat resistance andmechanical properties. These copolymers may be either the blockcopolymer or the random copolymer.

The polymerization catalyst which is used when polymerizing thepolypropylene resin to be used in the present invention is notparticularly limited. An organometallic complex with a performance asthe polymerization catalyst can be used. Usable examples thereof includethe polymerization catalyst generally referred to as Ziegler-Nattacatalyst and referred to as the metallocene catalyst or homogeneouscatalyst. Ziegler-Natta catalyst is an organometallic complex obtainedby using titanium, aluminum, and magnesium as nucleus elements andmodifying a part or all of them with an alkyl group. The metallocenecatalyst is an organometallic complex obtained by using transitionmetals such as zirconium, titanium, thorium, lutetium, lanthanum, andiron, or boron as nucleus elements and modifying them with acyclopentane ring. Alternatively, a combination of the organometalliccomplex and methylalumoxane may be used.

Among the polypropylene resins, the polypropylene resin which ispolymerized with the metallocene polymerization catalyst (hereinafterreferred to as mPP) is preferable. As compared mPP to the polypropyleneresin which is polymerized with a general Ziegler-Natta catalyst(hereinafter referred to as zPP), when both of the polypropylene resinsshowing the same melting point are compared, the mechanical strength ofmPP is higher and foamed resin beads having an excellent mechanicalstrength can be obtained. From the above-described viewpoint, themetallocene catalyst which includes a complex having an azulenyl ligandof a silylene bridge is particularly preferable among the metallocenecatalysts.

The crystalline polyolefin resin constituting the core layer (R) haspreferably a melting point (Tm) of 100 to 250° C., more preferably 110to 170° C., particularly preferably 120 to 160° C. from the viewpoint ofbalance between fusion bonding properties in molding in a mold and heatresistance.

The melting point (Tm) is a value determined as a temperature showingthe apex of an endothermic peak accompanying the melting of thepolyolefin resin on a DSC curve when the DSC curve is drawn by risingtemperature at a heating rate of 10° C./min with the DSC apparatus inaccordance with “when a melting temperature is measured after performinga constant heat treatment” described in JIS K7121 (1987) (both theheating rate and the cooling rate in controlling the condition of thespecimen are 10° C./min). When a plurality of endothermic peaks arepresent on the DSC curve, the temperature of apex of endothermic peakhaving the highest apex based on a baseline at the high temperature sideamong the endothermic peaks is defined as a melting point. As themeasuring apparatus, DSCQ1000 (manufactured by TA INSTRUMENTS) can beused.

In the present invention, the core layer (R) may contain additive agentssuch as catalyst neutralizers, lubricants, and crystal nucleatingagents. In this regard, it is desirable that the content is as low aspossible within the range which does not impair the object of thepresent invention. The additive amount of the additive agents variesdepending the type and intended use. The additive amount is preferably15 parts by weight or less based on 100 parts by weight of thecrystalline resin, more preferably 10 parts by weight or less, furtherpreferably 5 parts by weight or less, particularly preferably 1 part byweight or less.

Usable examples of the polyolefin resin constituting the covering layer(R) of the present invention include the crystalline polyolefin resinwhich has the melting point lower than that of the polyolefin resinconstituting the core layer (R) or the noncrystalline polyolefin resinwhich does not substantially show the melting point and has thesoftening point lower than the melting point of the polyolefin resinconstituting the core layer (R). The same type of polyolefin resinconstituting the covering layer (R) as the resin constituting the corelayer (R) can also be used.

In the polyolefin resin constituting the covering layer (R), examples ofthe crystalline polyolefin resin exhibiting a melting point includepolypropylene resin described above, low-pressure process high densitypolyethylene, low-pressure process low density polyethylene,high-pressure process low density polyethylene resin, linear low densitypolyethylene resin, ultra low density polyethylene resin andpolyethylene copolymers prepared from ethylene and monomers such asvinyl acetate, unsaturated carboxylic acid ester, unsaturated carboxylicacid, and vinyl alcohol. Examples of the noncrystalline polyolefin resininclude polyethylene rubbers such as ethylene-propylene rubber,ethylene-propylene-diene rubber, ethylene-acrylic rubber, chlorinatedpolyethylene rubber, and chlorosulfonated polyethylene rubber;polyolefin elastomer, and noncrystalline polypropylene resin.

With reference to the polyolefin resin constituting the covering layer(R), polypropylene resin (i.e., crystalline polyolefin resin),high-pressure process low density polyethylene resin, low-pressureprocess low density polyethylene, and linear low density polyethyleneresin are preferable among the polyolefin resins described above. Amongthe resins, the polyolefin resin polymerized using metallocenepolymerization catalyst is more preferable. The polyolefin resinpolymerized using the metallocene polymerization catalyst is furtherexcellent in fusion bonding properties with other polyolefin resinssince the content of a low molecular weight component is lower than thatof the polyolefin resin polymerized using Ziegler Natta polymerizationcatalyst. When the polyolefin resin polymerized using the metallocenepolymerization catalyst is used for the resin constituting the coveringlayer (R), the fusion bonding properties of foamed resin beads becomemore excellent. Further, the adhesive property of the core layer (R) tothe covering layer (R) becomes more excellent. Therefore, the moldedfoamed article obtained from the foamed resin beads is more excellent inmechanical properties. When the resin constituting the core layer (R) isthe polypropylene resin and the resin constituting the covering layer(R) is the polypropylene resin, the adhesive property of the core layer(R) and the covering layer (R) is excellent. Thus, the productivity isexcellent and the molded foamed article obtained by molding the foamedresin beads in a mold has an excellent mechanical strength, which ispreferable. When mPP is used, the above-described effect is increasedand the fusion bonding properties of the foamed resin beads are furtherexcellent, which is more preferable.

The melting point of the polyolefin resin constituting the coveringlayer (R) of the present invention needs to be lower than the meltingpoint of a crystalline polyolefin resin constituting the core layer (R)in the case that the polyolefin resin constituting the core layer (R) isa crystalline polyolefin resin. In the case that the polyolefin resinconstituting the core layer (R) is a noncrystalline resin, the softeningpoint of the polyolefin resin constituting the covering layer (R) of thepresent invention needs to be lower than the melting point of thepolyolefin resin constituting the core layer (R).

However, when the melting point or softening point of the polyolefinresin constituting the covering layer (R) is too lower than the meltingpoint of the polyolefin resin constituting the core layer (R), thefusion between the foamed resin beads is caused at the time of foamingand the blocking of the foamed resin beads can be occurred. Further, theheat resistance of the molded foamed article produced by molding thefoamed resin beads in a mold can be deteriorated. Even in the case thatthe melting point or softening point of the resin constituting thecovering layer (R) is low, when there is no difference between themelting points of the resin constituting the core layer (R) and theresin constituting the covering layer (R), the desired object of thepresent invention for improving the fusion bonding properties cannot beachieved without any damage is not given to the cell structure of thefoamed resin beads.

Based on this standpoint, when the polyolefin resin constituting thecovering layer (R) is the crystalline polyolefin resin, the crystallinepolyolefin resin to be used has the temperature difference [(A)−(B)]between the melting point (A) of the polyolefin resin constituting thecore layer (R) and the melting point (B) of the polyolefin resinconstituting the covering layer (R) being more than 0° C. and 80° C. orless. The temperature difference is preferably in the range of 1 to 80°C., more preferably in the range of 5 to 60° C., further preferably inthe range of 10 to 50° C., particularly preferably in the range of 15 to45° C.

On the other hand, when the polyolefin resin constituting the coveringlayer (R) is the noncrystalline polyolefin resin, the noncrystallinepolyolefin resin to be used has the temperature difference [(A)−(C)]between the melting point (A) of the polyolefin resin constituting thecore layer (R) and the softening point (C) of the polyolefin resinconstituting the covering layer (R) being more than 0° C. and 100° C. orless. The temperature difference is preferably in the range of 1 to 100°C., more preferably in the range of 5 to 60° C., further preferably inthe range of 10 to 50° C., particularly preferably in the range of 15 to45° C. From the viewpoint of handling and heat resistance of the moldedfoamed article to be obtained, the relationship of the melting point ofthe crystalline polyolefin resin and the softening point of thenoncrystalline polyolefin resin constituting the covering layer (R) withthe melting point of the crystalline polyolefin resin constituting thecore layer (R) satisfies the above-described range. The melting point ofthe crystalline polyolefin resin and the softening point of thenoncrystalline polyolefin resin constituting the covering layer (R) arepreferably 40° C. or more, more preferably 60° C. or more, furtherpreferably 80° C. or more, particularly preferably 90° C. or more.

In the present invention, the term “softening point” means a vicatsoftening point measured by the A50 method based on JIS K7206 (1999). Aspecimen for measurement (20 mm long×20 mm wide×3 mm thick) is producedby pressurizing an amorphous polyolefin resin at 230° C. 5 MPa so as notto contain bubbles. The obtained specimen is used for measurementwithout annealing treatment. As the measuring apparatus, HDT/VSPT testequipment MODEL TM-4123 (manufactured by Ueshima Seisakusho Co., Ltd.)and the like can be used.

If necessary, additive agents such as lubricants, catalyst neutralizers,and antioxidizing agents can be added to the covering layer (R) withinthe range which does not impair the object of the present invention. Theadditive amount varies depending on the type of additive agent and it ispreferably 15 parts by weight or less based on 100 parts by weight ofpolyolefin resin, more preferably 10 parts by weight or less, furtherpreferably 5 parts by weight or less, particularly preferably 1 part byweight or less.

In the present invention, the polymer type antistatic agents are blendedwith the covering layer (R), the blending amount is in the range of 10%by weight or more and less than 50% by weight based on the total weightof the covering layer (R). When the blending amount of the polymer typeantistatic agent is less than 10% by weight, the desired antistaticfunction cannot be obtained. On the other hand, when the blending amountis 50% by weight or more, the fusion bonding properties of the foamedresin beads are reduced and the surface condition of the molded foamedarticle is deteriorated. Thus, a good molded foamed article cannot beobtained. Further, when the blending amount is 50% by weight or more,the antistatic performance is hardly changed and any effect worth theblending amount cannot be obtained. Thus, the cost performance isworsened. From this viewpoint, the lower limit as to the blending amountof the antistatic agent to the covering layer (R) is preferably 12% byweight, more preferably 15% by weight. On the other hand, the upperlimit is preferably 40% by weight, more preferably 30% by weight.

In the present invention, the polymer type antistatic agent is blendedwith the covering layer (R) in the above described manner. From theviewpoint of importance to the antistatic properties, the molded foamedarticle obtained by blending the polymer type antistatic agent to thecore layer (R) within the range which does not disturb the foamingfoamed resin beads and the range which does not cause a large shrinkageof foamed resin beads can exhibit a further excellent antistaticperformance. When the cross section of the foamed resin beads has aportion in which the covering layer is not present or when the moldedfoamed article have the cutting surface obtained by cutting the moldedfoamed article, the antistatic agent is present in the core layer (E).Thus, the surface of the whole molded foamed article is a phasecontaining the antistatic agent, an excellent antistatic effect can beobtained.

When a small amount of the antistatic agent is blended to the core layer(R), the blending amount of the antistatic agent to be contained in thecovering layer (R) can be decreased without reducing the antistaticperformance. Therefore, the fusion bonding properties between foamedresin beads become more excellent. In this case, it is necessary to setthe blending amount of the antistatic agent to the core layer (R) to therange which does not disturb the foaming. The upper limit as to theblending amount of the antistatic agent to the core layer (R) is 15% byweight or less based on the total weight of the core layer (R),preferably 12% by weight or less, more preferably 8% by weight or less.

On the other hand, the lower limit as to the blending amount of theantistatic agent is preferably 5% by weight or more, more preferably 7%by weight or more from the viewpoint of obtaining a high antistaticperformance. When the antistatic agent is blended to the core layer (R),it is preferable to adjust the antistatic performance by making theblending amount of the antistatic agent to the core layer (R) lower thanthe blending amount of the antistatic agent of the covering layer (R)from the viewpoint of cost performance of the antistatic performance.

When the antistatic agent is blended to the core layer (R), themechanical strength of foamed resin beads tends to easily lowered. Thus,it is preferable to use mPP as the polyolefin resin constituting thecore layer (R). Particularly, mPP polymerized with the metallocenecatalyst which includes a complex having an azulenyl ligand of asilylene bridge among the metallocene catalysts is preferable.

On the other hand, from the viewpoint of the mechanical strength of themolded foamed article, it is preferable that the core layer (R) does notsubstantially contain the antistatic agent. In general, the mechanicalstrength of the antistatic agent in itself is lower than that of thepolyolefin resin to be used in the present invention. Additionally, themechanical strength of the base material in itself is easily lowered bymixing with different types of raw materials, and a reduction in thefoaming properties is easily caused. Thus, the mechanical strength ofthe foamed resin beads is lowered by blending the antistatic agent tofoamed resin beads. However, when foamed resin beads are producedwithout adding the antistatic agent to the core layer (R), the foamedresin beads have the mechanical properties equivalent to those ofgeneral foamed resin beads which do not include the antistatic agent.When the antistatic agent is not included in the core layer (R), thelower limit of the blending amount of the antistatic agent of thecovering layer (R) is preferably 20% by weight, more preferably 25% byweight, further preferably 28% by weight in order to obtain the desiredantistatic performance.

The term “not substantially contain” means that the blending amount ofthe antistatic agent is the blending amount in which foamed resin beadshaving foaming properties equivalent to those of the foamed resin beadswhich do not include the antistatic agent and mechanical properties canbe obtained and any antistatic performance is not exhibited. Usually, itis 3% by weight or less (0 is included).

The polymer type antistatic agent used in the present invention is aresin with a surface resistivity of less than 1×10¹²ω. Specifically, anionomer resin containing alkali metal selected from the group consistingof potassium, rubidium, and cesium as a metal ion or a resin containinghydrophilic resins such as polyether ester amide and polyether as a maincomponent are preferable. It is further preferable to use the resinblock-copolymerized with the polyolefin resin as the polymer typeantistatic agent in order to improve compatibility with the polyolefinresin constituting the foamed resin beads and obtain an effect whichsuppresses the reduction of the mechanical property caused by adding theantistatic agent.

Particularly preferable examples of the polymer type antistatic agentinclude compositions described in JP-A Nos. 3-103466 and 2001-278985.

A composition described in JP-A No. 3-103466 includes the followingcomponents:

(I) a thermoplastic resin;

(II) polyethylene oxide or a block copolymer which contains 50% byweight or more of polyethylene oxide block component; and

(III) metal salt which is dissolved in the polyethylene oxide blockcomponent described in (II).

Further, a composition described in JP-A No. 2001-278985 is a blockcopolymer having a number average molecular weight (Mn) of 2000 to 60000which has a structure in which a block (a) of polyolefin is repeatedlyand alternatively bonded to a block (b) of a hydrophilic resin having avolume resistivity value of 1×10⁵ to 1×10¹¹ Ω·cm. The block (a) and theblock (b) have a structure in which they are bonded repeatedly andalternatively via at least one bond selected from the group consistingof an ester bond, an amide bond, an ether bond, an urethane bond, and animide bond.

The number average molecular weight of the polymer type antistatic agentto be used herein is preferably 2000 or more, more preferably in therange of 2000 to 100000, further preferably 5000 to 60000, particularlypreferably 8000 to 40000. In this regard, the upper limit of the numberaverage molecular weight of the polymer type antistatic agent isapproximately 500000. When the number average molecular weight of thepolymer type antistatic agent is in the above described range, theantistatic performance is not influenced by environment such as humidityand is more stably exhibited. Further, the transfer of the antistaticagent to packaging products is not observed.

The number average molecular weight is determined using high temperaturegel permeation chromatography. For example, when the polymer typeantistatic agents containing polyether ester amide and polyether as maincomponents are used, the number average molecular weight is a valuemeasured under conditions where orthodichlorobenzene is used as asolvent, a sample concentration is 3 mg/ml, polystyrene is used as areference substance, and a column temperature is 135° C. In this regard,the type of the solvent and the column temperature are suitably changeddepending on the type of polymer type antistatic agent.

The melting point of the polymer type antistatic agent is preferably 70to 270° C., more preferably 80 to 230° C., further preferably 80 to 200°C., particularly preferably 90 to 180° C. from the viewpoint of theantistatic functions. It is desired that the difference between themelting point of the polyolefin resin constituting the covering layer(R) and the melting point of the polymer type antistatic agent or thedifference between the softening point of the polyolefin resinconstituting the covering layer (R) and the melting points of thepolymer type antistatic agent (when the polyolefin resin constitutingthe covering layer (R) does not have the melting point) is preferably150° C. or less, particularly preferably 100° C. or less from theviewpoint of dispersibility to the polyolefin resin at the time ofkneading and fusion bonding properties at the time of molding.

The melting point of the polymer type antistatic agent can be measuredby the method based on JIS K7121 (1987). That is, pretreatment isperformed under the condition of the controlling the condition of thespecimen (2) of specimen based on JIS K7121 (1987) (where the coolingrate is 10° C./min) and a DSC curve is obtained by rising temperature ata heating rate of 10° C./min. The temperature of the apex of theobtained endothermic peak is defined as a melting point. When two ormore endothermic peaks appear, the apex of the main endothermic peak(peak with the largest area) is defined as the melting point. Whenanother peak with a peak area of 80% or more to the peak area of thepeak with the largest area is present, an arithmetical average value ofa temperature of the apex of the peak and a temperature of the apex ofthe peak with the largest area is used as the melting point.

The polymer type antistatic agents may be respectively used alone or incombination. In this regard, the polymer type antistatic agents areavailable as commercialized products (for example, SD100, manufacturedby DU PONT-MITSUI POLYCHEMICALS CO., LTD., PELESTAT 300, PELESTAT 230,and PELESTAT 3170, manufactured by Sanyo Chemical Industries, Ltd,).

The composite resin beads consist of the core layer (R) and the coveringlayer (R) according to the present invention can be produced using theco-extrusion die described in JP-B Nos. 41-16125, 43-23858, 44-29522,and JP-A No. 60-185816. Generally, composite resin beads are obtained bythe steps of melt-kneading necessary polyolefin resin components withthe polymer type antistatic agent, and if necessary the additive agent,by the extruder for the core layer, melt-kneading necessary polyolefinresin components with the polymer type antistatic agent, and ifnecessary the additive agent, by the extruder for the covering layer,allowing each of the melt-kneaded products to be joined in theco-extrusion die connected to end of an extruder for the core layer andend of an extruder for the covering layer, forming a sheath-core type ofcomposite body which includes the cylindrical core layer (R) in anon-foaming state and the covering layer (R) in the non-foaming statewhich covers the side surface of the core layer (R), extruding thecomposite body from a small hole of the die into a strand shape, andcutting particles with a pelletizer so as to have a predetermined weightof the particles. Hereinafter, such a composite structure may bereferred to as a “sheath-core” structure.

The weight ratio (% by weight) of the core layer (R) of the compositeresin beads to the covering layer (R) is preferably 99.5:0.5 to 98:20,more preferably 98:2 to 80:20, further preferably 96:4 to 90:10.

When the weight ratio of the covering layer (R) is too small, thethickness of the covering layer (E) becomes too thin. As a result,effect of improvement in fusion bonding properties cannot be obtained,the fusion bonding between foamed resin beads is easily insufficient.Further, the antistatic effect may become insufficient. On the otherhand, when the weight ratio of the covering layer (R) is too large, themechanical properties of the covering layer (E) of foamed resin beads inthemselves tend to be reduced, since the resin having the melting pointor softening point lower than the melting point of the core layer (R) isused for the covering layer (R). Further, the covering layer (R) foamseasily. When the ratio of the covering layer (R) to all of the foamedresin beads is increased, the mechanical properties of the molded foamedarticle tend to be reduced. That is, when the weight ratio of the corelayer (R) and the covering layer (R) is within the range describedabove, the antistatic properties and fusion bonging properties areexcellent. Further, as for the molded foamed article obtained by moldingfoamed resin beads in a mold, cells are not present near the fusedinterface of the foamed resin beads, the fusing strength between thefoamed resin beads is particularly strong, and the molded foamed articleis excellent in mechanical strength.

It is preferable that the thickness of the covering layer (R) of thecomposite resin beads is thin. This is because cells in the coveringlayer (E) are hardly generated when the composite resin beads are foamedand expanded. However, when the thickness is too thin, effect ofimprovement in fusion bonding properties between foamed resin beads isreduced and further it becomes difficult to sufficiently cover the corelayer (R) in itself. When the thickness of the covering layer (R) is toothick, cells are easily produced in the covering layer (E) at the timeof foaming and expanding the composite resin beads, which can cause areduction in the mechanical strength of the molded foamed article.Therefore, the thickness of the covering layer (E) of the foamed resinbeads is preferably 0.1 to 200 μm, more preferably 0.5 to 50 μm. Thethickness of the covering layer (R) at the stage of composite resinbeads needs to be adjusted so that the thickness of the covering layer(E) of the foamed resin beads is within the above described range.Although the thickness of the covering layer (R) varies depending on thesize of composite resin beads and the expansion ratio, it is preferably5 to 500 μm, more preferably 10 to 100 μm.

The foamed resin beads of the present invention are produced bydispersing the composite resin beads composed of the core layer (R) andthe covering layer (R) into an aqueous medium (generally water) in aclosable container (e.g. autoclave) which can be pressurized, adding adispersing agent, injecting a predetermined amount of a blowing agentand pressurizing, stirring under heating for a predetermined time,impregnating the composite resin beads with the blowing agent, and thenreleasing the aqueous medium and contents into a low-pressure area withan internal container pressure to allow them to foam and expand. At thetime of the release, it is preferable that the contents are released byapplying back pressure to the container. Particularly, when foamed resinbeads having a high expansion ratio are produced, the foamed resin beadscan be obtained by curing the foamed resin beads obtained in theabove-described manner (a usual curing step) at atmospheric pressure,charging the foamed resin beads into a closable container which can bepressurized, pressurizing with a gas under pressure such as air,performing an operation for increasing the pressure in the foamed resinbeads, taking out the foamed resin beads from the container, heatingthem using heating medium such as steam and hot air (hereinafterreferred to as two-stage expanding).

In the present invention, the blowing agent is not particularly limited.For example, hydrocarbons such as butane, pentane, and hexane;halogenated hydrocarbons such as trichlorofluoromethane,dichlorofluoromethane, tetrachlorodifluoroethane, and dichloromethane;inorganic gases such as carbon dioxide, nitrogen, and air; and water maybe used alone or two or more of them may be used in combination. Amongthese blowing agents, it is preferable to use a physical blowing agentcontaining inorganic physical blowing agents such as carbon dioxide,nitrogen, and air as main components. It is more preferable to usecarbon dioxide. In the present invention, the term “containing inorganicphysical blowing agents as main components” means that the content ofthe inorganic physical blowing agent is 50 mol % or more based on 100mol % of the physical blowing agent, preferably 70 mol % or more, morepreferably 90 mol % or more. When an organic physical blowing agent isused, it is preferable to use normal butane, isobutane, normal pentane,and isopentane as the organic physical blowing agent from the viewpointof compatibility with the polyolefin resin and blowing properties.

The additive amount of the physical blowing agents is appropriatelyadjusted according to the type of blowing agent, the blending amount ofantistatic agent, and the apparent density of the desired foamed resinbeads and cannot be generally specified. For example, when carbondioxide is used as the physical blowing agent, the additive amount is0.1 to 30 parts by weight based on 100 parts by weight of compositeresin beads, preferably 0.5 to 15 parts by weight, more preferably 1 to10 parts by weight.

Examples of the dispersing agent include hardly water-soluble inorganicsubstances such as aluminum oxide, tri calcium phosphate, magnesiumpyrophosphate, zinc oxide, kaolin, and mica; and water-soluble polymerprotective colloid agents such as polyvinyl pyrrolidone, polyvinylalcohol, and methyl cellulose. Further, anionic surfactants such assodium dodecylbenzenesulfonate and sodium alkane sulfonate can be used.

The resulting foamed resin beads have a composite structure in which thecore layer (E) having a microcells in a foaming state and the coveringlayer (E) which is formed on the surface of the core layer. It ispreferable that the apparent density of the foamed resin beads is 10 to180 kg/m³ and the average cell diameter is 50 to 900 μm. It is furtherpreferable that the average cell diameter is 100 to 300 μm. The foamedresin beads has a ratio of an apparent density of foamed resin beadsafter pressurizing under constant conditions (with a compressed air at30° C. and 0.2 MPa (G) for 24 hours) and leaving under an ordinarypressure at 23° C. for 24 hours to an apparent density of foamed resinbeads before the pressurization is in the range of 0.8 to 1.0 (apparentdensity after pressurization/apparent density before pressurization).The shrinkage immediately after foaming is small, the control of densityof the foamed resin beads is easy, and the bridge can be hardlygenerated at the time of transfer. In the case of such foamed resinbeads, the time required for a step of applying an internal pressurewhich is needed for a two-stage expanding step or a pressure molding isshortened. Further, the foamed resin beads have a characteristic inwhich the mechanical properties are hardly decreased by the shrinkagehistory.

The apparent density of the foamed resin beads is measured by thefollowing manner. A group of the foamed resin beads (weight, W(g)) isimmersed in a graduated cylinder containing water using a wire net andthe like. The volume V (L) of the group of the foamed resin beads iscalculated from the rising portion of water level. The value (W/V)obtained by dividing the weight (w) of the group of the foamed resinbeads by the volume (V) of the group of the foamed resin beads isconverted to kg/m³.

In the foamed resin beads of the present invention, it is preferablethat one or more peaks of the endothermic peak (high temperature peak)are present at the high temperature side rather than the apex of theendothermic peak (intrinsic peak) inherent to the crystalline polyolefinresin constituting the core layer (R) in the DSC curve obtained by heatflux differential scanning calorimetry (hereinafter simply referred toas “DSC measurement”). The foamed resin beads have a high closed cellratio and are suitable for fusion bonding.

The calorific value of a high temperature peak to be needed largelyvaries depending on the type of resin constituting foamed resin beadsand further varies depending on the ratio of the core layer (R) to thecovering layer (R) or the change in the amount of the additive agent.Therefore, although it is not necessarily appropriate to suggest it, itis preferably 50 J/g or less.

The calorific value of a high temperature peak of foamed resin beads isthe calorific value of an endothermic peak b (high temperature peak)which appears at the temperature side higher than the temperature inwhich an endothermic peak a (intrinsic peak) inherent to the resinconstituting foamed resin beads appears in the first DSC curve (shown inFIG. 1) which is obtained when 1 to 3 mg of foamed resin beads areheated up from room temperature (10 to 40° C.) to 220° C. at a heatingrate of 10° C./min with the DSC apparatus and corresponds to an area ofthe high temperature peak b. Specifically, it can be determined in thefollowing manner.

FIG. 1 is an example of the foamed resin beads when polypropylene resinis used as a base resin.

First, a straight line (α-β) which connects a point α corresponding to80° C. on the DSC curve with a point β on the DSC curve corresponding toa melting end temperature T of foamed resin beads is drawn. Next, astraight line parallel to the vertical axis in the graph is drawn fromthe point γ on the DSC curve corresponding to the valley portion betweenthe intrinsic peak a and the high temperature peak b and a point thatcrosses the straight line (α-β) is σ. The area of the high temperaturepeak b is an area of a portion (shaded portion in FIG. 1) surrounded bya curve of the high temperature peak b on the DSC curve, a line segment(σ-β), and a line segment (γ-σ) and it corresponds to the calorificvalue of a high temperature peak. In this regard, the term “melting endtemperature T” means a point of intersection of the DSC curve at theside of high temperature of the high temperature peak b and the baselineat the high temperature side.

The total (the amount of melting heat as to all of the foamed resinbeads) of the calorific value of a high temperature peak and thecalorific value of the intrinsic peak corresponds to an area of aportion surrounded by the straight line (α-β) and the DSC curve.

In this regard, the high temperature peak b appears in the first DSCcurve, however, it does not appear in the second DSC curve obtained whenthe temperature is once lowered from 220° C. to around 40° C. (40 to 50°C.) at 10° C./min after obtaining the first DSC curve and thetemperature is again increased to 220° C. at 10° C./min. As shown inFIG. 2, only the intrinsic peak a of the resin constituting foamed resinbeads is observed.

When three or more endothermic peaks appear in the first DSC curve, forexample, a mixture of a polyolefin resin containing two or more baseresins of foamed resin beads, more specifically, foamed resin beadsobtained by foaming and expanding resin beads having the compositestructure in which the covering layer is the polyethylene resin and thecore layer is the polypropylene resin are listed. In this case, the hightemperature peak is not observed in the second DSC curve. Comparison ofthe first DSC curve with the second DSC curve using the result allowsfor determining which peak is the high temperature peak.

As the measuring apparatus, DSCQ1000 (manufactured by TA INSTRUMENTS)and the like can be used.

A molding method known in itself can be used as a method for producingthe molded foamed article using the foamed resin beads of the presentinvention.

For example, a reduced-pressure molding method (e.g. JP-B No. 46-38359)which involves the steps of charging foamed resin beads to a moldingcavity using a pair of molds for molding conventional foamed resin beadsunder atmospheric pressure or reduced pressure, closing the mold,compressing so as to reduce the volume in the mold cavity by 5 to 70%,then supplying heating medium such as steam to the mold, heating, andfuse-bonding the foamed resin beads is known. Alternatively, acompression molding method (e.g. JP-B No. 51-22951) which involves thesteps of pre-pressurizing foamed resin beads by a pressurized gas toincrease the pressure in the foamed resin beads, improving a secondaryexpanding property of the foamed resin beads, maintaining the secondaryexpanding property, charging foamed resin beads to a molding cavityunder atmospheric pressure or reduced pressure, closing the mold, thensupplying heating medium such as steam to the mold, heating, andfuse-bonding the foamed resin beads is used for molding.

Further, a compression filling molding method (JP-B No. 4-46217) whichinvolves the steps of pressurizing the cavity by compressed gas at apressure higher than atmospheric pressure, charging foamed resin beadspressurized at a pressure higher than that of the cavity, then supplyingheating medium such as steam to the cavity, heating, and fuse-bondingthe foamed resin beads can be used for molding. In addition, a normalpressure filling method (JP-B No. 6-49795) which involves the steps ofcharging foamed resin beads having a high level of secondary expandingcapability which are obtained in specific conditions to a cavity of apair of molds under atmospheric pressure or reduced pressure, thensupplying heating medium such as steam to the cavity, heating, andfuse-bonding the foamed resin beads or a method combined with theabove-described method (JP-B No. 6-22919) can be used for molding.

The fusion bonding rate of the molded foamed article obtained by moldingin a mold the foamed resin beads of the present invention is preferably75% or more, more preferably 80% or more, further preferably 85% ormore, particularly preferably 90% or more. The molded foamed articlewith a high fusion rate is excellent in mechanical strength,particularly flexural strength.

The term “fusion bonding rate” used here in means a material failurerate based on the number of foamed resin beads on the fracture surfacewhen the molded foamed article is bent and fractured. The material atthe non-fused portion is not fractured and the interface of foamed resinbeads is separated.

The molded foamed article obtained by molding in a mold the foamed resinbeads in the present invention is excellent in fusion properties,mechanical properties and, particularly compressive strength. Thesurface resistivity of the molded foamed article is less than 1×10¹⁴Ω,which shows an excellent antistatic property.

The apparent density of the molded foamed article of the presentinvention is preferably 10 to 180 kg/m³. When the apparent density iswithin the range described above, the foamed molded body has anexcellent balance between lightweight properties and mechanicalproperties. In order to determine the apparent density, the valueobtained by dividing the weight of the molded foamed article by thevolume calculated from an outside dimension of the molded foamed articleis converted to [kg/m³].

The term “surface resistivity” in the present invention means a valuemeasured based on JIS K 6271 (2001). Specifically, three pieces ofspecimens (100 mm length×100 mm width×thickness: the thickness of themolded body) cut from the central portion of the molded foamed articleare prepared. The specimens are left under conditions of 23° C. and ahumidity of 50% RH for 24 hours. Thereafter, a voltage of 500V isapplied to each of the specimens under conditions of 23° C. and 50% RH.After 30 seconds, each of the electric current values is measured andeach of the surface resistivities is calculated. Each surfaceresistivity is determined by arithmetic-averaging the surfaceresistivities for each of the specimens. As the measuring apparatus,Hiresta MCP-HT450 (manufactured by Mitsubishi Chemical Corporation) canbe used.

EXAMPLES

Crystalline polyolefins used in Examples and Comparative examples areshown in the following Table 1.

TABLE 1 Comonomer MFR* Melting Abbrevi- amounts g/10 point ationcatalysts Based resins comonomers % by weight min (° C.) Resin 1Metallocene type Propylene-ethylene Ethylene 0.5 9.0 142 randomcopolymer Resin 2 Ziegler Natta type Propylene-ethylene Ethylene 2.7 7.0142 random copolymer Resin 3 Ziegler Natta type Propylene-ethyleneEthylene 1.0 8.0 155 random copolymer Resin 4 Ziegler Natta typePropylene homopolymer — — 5.0 162 Resin A Metallocene typePropylene-ethylene Ethylene 2.8 7.0 125 random copolymer Resin B ZieglerNatta type Propylene-ethylene Ethylene 1.6 7.0 135 random copolymerResin C Metallocene type Linear low density — — 2.0 100 polyethylene*MFR means a melt mass-flow rate measured by a test method A based onJIS K 7210 (1999). MRFs of the resins 1 to 4 as well as the resins A andB are values measured under conditions of a test temperature of 230° C.and a load of 2.16 kg. MFR of the resin C is a value measured underconditions of a test temperature of 190° C. and a load of 2.16 kg.

The polypropylene resin for covering layer formation used in Examples“amorphous PP, softening point: 64° C.” is noncrystalline polypropyleneresin (grade name: VERSIFY 3200, manufactured by Dow Plastics).

As for polymer type antistatic agents used in Examples, “PEO” representsa polyolefin-polyethylene oxide block copolymer type (trade name:PELESTAT 230, manufactured by Sanyo Chemical Industries Co., Ltd.) and“PEEA” represents a polyether ester amide type (trade name: PELESTAT3170, manufactured by Sanyo Chemical Industries Co., Ltd.).

[Preparation of Composite Resin Beads Including Core Layer and CoveringLayer]

An apparatus to which a die for multilayer strand formation was attachedat the outlet side of an extruder for the core layer having an innerdiameter of 50 mm and an extruder for the covering layer having an innerdiameter of 30 mm was used. The polyolefin resins for constituting thecore layer shown in Tables 2 to 4 and the polyolefin resins forconstituting the covering layer were supplied to the extruder for thecore layer and the extruder for the covering layer, respectively at theratios shown in Tables 2 to 4, which was melt-kneaded. The respectivemelt-kneaded resins were introduced into the die for forming amultilayer strand and were joined in the die, which was extruded througha small hole of the die attached to the outlet side of the extruder as astrand formed into a two-layer structure (sheath-core structure). Then,the extruded strand was water-cooled, which was cut with a pelletizer soas to be a mean weight of almost 1 mg, followed by drying. Then,composite resin beads were obtained.

In this regard, zinc borate was supplied to the polyolefin resin forconstituting the core layer as a cell adjusting agent with masterbatchso that the content of zinc borate should be 500 weight ppm. Theantistatic agents shown in Tables 2 to 4 were blended with thepolyolefin resin for constituting the covering layer at a predeterminedamount so as to have the blending amount shown in Tables 2 to 4. Whenthe antistatic agents were added to the core layer, the antistaticagents shown in Tables 2 to 4 were blended with the polyolefin resin forconstituting the core layer at a predetermined amount so as to have theblending amount shown in Table. The resulting mixtures were supplied torespective extruders.

In addition, the content of the antistatic agent in the covering layerand/or core layer of the resin particles is equal to the blended amountof the antistatic agent to them.

[Preparation of Foamed Composite Resin Beads]

1 kg of the composite resin beads obtained above and 3 L of water(dispersion medium) were charged to a 5 L autoclave. 0.3 part by weightof kaolin as the dispersing agent, 0.004 part by weight of surfactant(sodium alkylbenzene sulfonate), and 0.01 part by weight of aluminumsulfate based on 100 parts by weight of composite resin beads wererespectively added to the dispersion medium. The blowing agents shown inTables 2 to 4 were injected into the autoclave so as to have theinternal autoclave pressures shown in Tables 2 to 4. The resultingproduct was heated to a “foaming temperature”, shown in Tables 2 to 4,with stirring, which was maintained at the same temperature forpredetermined time and the calorific value of a high temperatureendothermic peak was adjusted. Thereafter, the contents in the autoclavewere released with water under atmospheric pressure to produce foamedresin beads.

The foamed resin beads having a high expansion ratio (low density foamedresin beads) of Example 18 were prepared by using the two-stageexpanding method. In particular, foamed resin beads having an apparentdensity of 45 kg/m³ were first produced and then the usual curing stepat atmospheric pressure was carried out. Thereafter, the other autoclavewas filled up with the foamed resin beads, which was subjected to apressurizing step, followed by heating with steam. Then, compositefoamed resin beads having an apparent density of 18 kg/m³ were prepared.

[Preparation of Molded Foamed Article]

The foamed composite resin beads obtained in the above described mannerwere charged into a molding cavity (250 mm length×200 mm width×50 mmthickness), which was molded with steam at the molding pressures shownin Tables 2 to 4. In this regard, the steam pressure in which the moldedfoamed article was not greatly shrunk and showed the highest fusion ratewas used as the molding pressure. When the pressure was higher than themolding pressure, the molded foamed article was greatly shrunk or thefusion rate was reduced. As a result, a good molded foamed article wasnot obtained. The pressure was discharged after the end of heating. Themolded foamed article was water-cooled until the surface pressure whichderives from the expanding force of the molded foamed article wasreduced to 0.04 MPa (G). The mold was opened and the molded foamedarticle was taken out from the mold. The obtained molded foamed articlewas cured in an oven at 80° C. for 12 hours and a molded polypropyleneresin foamed article was obtained. The physical properties of the moldedfoamed article thus obtained were shown in Tables 2 to 4.

Evaluation of physical properties of the foamed resin beads and themolded foamed article was performed in the following manner.

[Antistatic Properties]

As for the antistatic performance of the molded foamed article, thesurface resistivity was measured and evaluated. After curing the moldedfoamed article under conditions of a temperature of 23° C. and ahumidity of 50% RH for one day, the surface resistivity was measured bythe above described method based on JIS K 6271 (2001). The surfaceresistivity of the “skin surface” in Table was a value obtained bycutting a specimen for measurement in a rectangular parallelepiped shapehaving a dimension of 100 mm length, 100 mm width and 50 mm thickness(thickness of the molded foamed article) from near the central portionof the molded foamed article and measuring the skin surface of thespecimen. Alternatively, the surface resistivity of the “cut surface”was a value obtained by cutting a specimen for measurement in arectangular parallelepiped shape having a dimension of 100 mm length,100 mm width and 50 mm thickness (thickness of the molded foamedarticle), removing 10 mm of the skin surface in the thickness directionfrom one skin surface to form a specimen for measurement, and measuringthe surface from which the skin surface of the specimen for measurementwas removed. As the measuring apparatus, Hiresta MCP-HT450 (manufacturedby Mitsubishi Chemical Corporation) was used.

[Fusion Bonding Properties]

As for the fusion bonding performance of the molded foamed article, thefusion bonding rate was measured and evaluated. The fusion bonding ratewas determined by following method. The molded foamed article was bentand fractured. The number C1 of the foamed resin beads present on thefracture surface and the number C2 of the destroyed beads wasdetermined. A material failure rate was calculated as a percentage ofthe destroyed beads (C2/C1×100). The above described measurement wasperformed 5 times using different specimens. Respective material failurerates were found and an arithmetic average value thereof was defined asa fusion rate.

[Mechanical Strength of Molded Foamed Resin Article]

50% compressive stress of the molded foamed article was measured and themechanical strength of the molded foamed article was evaluated. Aspecimen was cut in a rectangular parallelepiped shape having adimension of 100 mm length, 100 mm width and 25 mm thickness (there wasno skin surface at the time of molding.) from the central portion of themolded foamed article. Then, a compression velocity relative to thespecimen was 10 mm/min and the load at 50% strain was determined basedon JIS K 6767 (1999). The obtained value was divided by a pressurereceived area of the specimen to determine 50% compressive stress (kPa).

[Shrink Properties of Foamed Resin Beads]

A compressed air was injected into a pressure-resistant vesselcontaining foamed resin beads so that the pressure in the vessel had agage pressure of 0.2 MPa, which was stored at 30° C. for 24 hours.Thereafter, the pressure was released and the foamed resin beads weretaken out from the vessel. The foamed resin beads taken out were left ina constant temperature and humidity bath at 23° C. and a relativehumidity of 50% RH under an ordinary pressure for 24 hours. A ratio ofthe apparent density of foamed resin beads after pressurization to theapparent density of foamed resin beads before pressurization (apparentdensity after pressurization/apparent density before pressurization) wasfound, which was defined as a shrinkage ratio.

[Apparent Density of Foamed Resin Beads]

A group of the foamed resin beads (weight, W(g)) was immersed in agraduated cylinder containing water using a wire net. The volume V (L)of the group of the foamed resin beads was calculated from the scale ofthe rising portion of water level. The value (W/V) obtained by dividingthe weight W of the group of the foamed resin beads by the volume V wasconverted to [kg/m³].

[Apparent Density of Molded Foamed Article]

The value obtained by dividing the weight of the molded foamed articleby the volume calculated from an outside dimension of the molded foamedarticle was converted to [kg/m³].

[Average Cell Diameter of Foamed Resin Beads]

The average cell diameter of foamed resin beads was measured by thefollowing method. A foamed bead was cut into nearly equal halves andcross-section was photographed using an electron microscope. On thephotograph, four straight lines each passing the center of thecross-section were drown in a radial pattern.

A total N (piece) of the number of cells crossing the four straightlines was calculated. A total L of the length of each four straight line(μm) was calculated. The value obtained by dividing the total L of thestraight lines by the total N of the number of cells was defined (L/N)as an average cell diameter of the foamed resin beads.

[Calorific Value of an Endothermic Peak]

DSC measurement of the foamed resin beads was performed by the abovedescribed method. The calorific value of the endothermic peak at a hightemperature side (high temperature peak) of the foamed beads as well asthe calorific value of the endothermic peak as to all of the foamedresin beads were determined. As the measuring apparatus, DSCQ1000(manufactured by TA INSTRUMENTS) was used.

(1) Comparative example 1 shows foamed resin beads with an apparentdensity of 60 kg/cm³ in which the foamed resin beads had a usual singlelayer structure, a resin 2 (zPP with a melting point of 142° C.) wasused as the polyolefin resin constituting foamed resin beads(corresponding to the core layer of the sheath-core structure), and 10%by weight of PEEA was mixed as the antistatic agent. The molded foamedarticle produced from the foamed resin beads had poor fusion propertiesand the desired antistatic properties were not obtained, either.

(2) Comparative example 2 shows foamed resin beads in which the blendingamount of the antistatic agent was increased to 15% by weight in thefoamed resin beads of Comparative example 1. The foamed resin beads weresignificantly shrunk immediately after foaming and expanding. In thecase of the molded foamed article produced by molding in a mold thefoamed resin beads at a molding pressure of 0.34 MPa, the desiredantistatic performance was obtained. However, fusion properties weresignificantly reduced. The molding pressure was set to 0.36 MPa in orderto improve the fusion properties. However, the molded foamed article wassignificantly shrunk by excessive heating and a good molded foamedarticle was not given.

(3) Comparative example 3 shows foamed resin beads in which thepolyolefin resin was changed to a resin 1 (mPP with a melting point of142° C.) and the antistatic agent was changed from PEEA (polyether esteramid type) to PEO (polyolefin-polyethylene oxide block copolymer type)in the foamed resin beads of Comparative example 1. In the case of themolded foamed article obtained from the foamed resin beads, the desiredantistatic performance was obtained, however, the fusion properties werestill insufficient. As with Comparative example 2, the molded foamedarticle was significantly shrunk by excessive heating and a good moldedfoamed article could not be produced when the molding pressure wasincreased.

(4) Comparative example 4 shows composite foamed resin beads with anapparent density of 60 kg/cm³ in which foamed resin beads had asheath-core structure, the resin 1 (mPP with a melting point of 142° C.)was used as the resin of the core layer (R), the same resin as that ofthe core layer (R) was used as the resin of the covering layer (R), andthe antistatic agent was added to the covering layer (R) and the corelayer (R) in the same manner as described in Example 6. Even when thefoamed resin beads had the sheath-core structure, there was notemperature difference between the melting point of the covering layer(R) and the melting point of the core layer (R). Thus, the moldedarticle obtained from the foamed resin beads was inferior in fusionproperties. As with Comparative example 2, the molded foamed article wassignificantly shrunk by excessive heating and a good molded foamedarticle could not be produced when the molding pressure was increased.

(5) Comparative example 5 shows the foamed particles produced in thesame manner as described in Example 1 except that the blending amount ofthe antistatic agent for the covering layer (R) was set to 5% by weight.The blending amount of the antistatic agent for the covering layer (R)was too low and thus the desired antistatic performance in the moldedarticle obtained from the foamed resin beads were not given.

(6) Comparative example 6 shows the foamed resin beads produced in thesame manner as described in Example 1 except that the blending amount ofthe antistatic agent for the covering layer (R) was set to 50% byweight. The blending amount of the antistatic agent for the coveringlayer (R) was too high and thus the molded article obtained from thefoamed resin beads was inferior in fusion properties. As withComparative example 2, the molded foamed article was significantlyshrunk by excessive heating and a good molded foamed article could notbe produced when the molding pressure was increased.

(7) Comparative example 7 shows the foamed resin beads produced in thesame manner as described in Example 6 except that the antistatic agentwas added to the core layer (R) without adding the antistatic agent tothe covering layer (R). Even when the antistatic agent was added to thecore layer (R) of the foamed resin beads, the molded article obtainedfrom the foamed resin beads could not construct a network of theantistatic agent because the antistatic agent was not added to thecovering layer (R). As a result, the antistatic performance was inferiorto that of the molded article obtained from the foamed resin beads ofComparative example 3 without the covering layer (E).

TABLE 2 Example Example Example Example Example 1 2 3 4 5 ManufacturingResin beads structure — Sheath- Sheath- Sheath- Sheath- Sheath-conditions core core core core core Cover- Poly- kinds — Resin A Resin AResin A Resin A Resin A ing olefin Melting ° C. 125 125 125 125 125layer resin point Anti- kinds — PEO PEO PEO PEO PEO static Blending % by15 20 30 40 15 agent amount weight Ratio % by 5 5 5 5 5 weight CorePoly- kinds — Resin 1 Resin 1 Resin 1 Resin 1 Resin 1 layer olefinMelting ° C. 142 142 142 142 142 resin point Anti- kinds — — — — — PEOstatic Blending % by 0 0 0 0 5 agent amount weight Ratio % by 95 95 9595 95 weight Melting point difference ° C. 17 17 17 17 17 (corelayer-covering layer) Blowing kinds — CO₂ CO₂ CO₂ CO₂ CO₂ agent InternalMPa (G) 2.65 2.65 2.65 2.65 2.45 pressure Foaming temperature ° C. 145.5145.5 145.5 145.5 145.5 Physical Apparent density kg/m³ 60 60 60 60 60properties of Average cell diameter μm 150 150 150 150 200 foamed resinShrinking ratio — 1.00 1.00 1.00 1.00 1.00 beads Calorific value of ahigh J/g 16 16 16 16 15 temperature peak Calorific value as to all ofJ/g 80 79 79 79 77 the foamed resin beads Manufacturing Molding pressureMPa (G) 0.28 0.28 0.28 0.32 0.28 conditions Physical Apparent densitykg/m³ 67 67 67 67 67 properties Fusion rate % 100 100 100 80 100 ofmolded Surface Skin Surface Ω 4.5 × 10¹³ 4.6 × 10¹² 1.9 × 10¹² 1.2 ×10¹² 3.0 × 10¹³ foamed resistivity Cut surface Ω 5.5 × 10¹³ 9.4 × 10¹²3.0 × 10¹² 2.0 × 10¹² 1.6 × 10¹³ article 50% compressive stress kPa 640640 640 640 620 Example Example Example Example Example 6 7 8 9 10Manufacturing Resin beads structure Sheath- Sheath- Sheath- Sheath-Sheath- conditions core core core core core Cover- Poly- kinds Resin AResin A Resin A Resin B Resin C ing olefin Melting 125 125 125 135 100layer resin point Anti- kinds PEO PEO PEO PEO PEO static Blending 15 1520 30 30 agent amount Ratio 5 5 5 5 5 Core Poly- kinds Resin 1 Resin 1Resin 1 Resin 1 Resin 1 layer olefin Melting 142 142 142 142 142 resinpoint Anti- kinds PEO PEO PEO — — static Blending 10 15 7.5 0 0 agentamount Ratio 95 95 95 95 95 Melting point difference 17 17 17 7 42 (corelayer-covering layer) Blowing kinds CO₂ CO₂ CO₂ CO₂ CO₂ agent Internal2.25 2.05 2.35 2.65 2.65 pressure Foaming temperature 145.5 145.5 145.5145.5 145.5 Physical Apparent density 63 67 60 60 60 properties ofAverage cell diameter 250 400 200 150 150 foamed resin Shrinking ratio0.96 0.89 1.00 1.00 1.00 beads Calorific value of a high 15 14 15 16 16temperature peak Calorific value as to all of 70 67 75 82 78 the foamedresin beads Manufacturing Molding pressure 0.26 0.26 0.26 0.32 0.28conditions Physical Apparent density 67 67 67 67 67 properties Fusionrate 100 100 100 80 100 of molded Surface Skin Surface 1.0 × 10¹² 7.8 ×10¹⁰ 2.7 × 10¹² 2.0 × 10¹² 1.5 × 10¹² foamed resistivity Cut surface 2.9× 10¹² 6.8 × 10¹⁰ 3.5 × 10¹² 3.0 × 10¹² 2.6 × 10¹² article 50%compressive stress 600 570 610 640 640

TABLE 3 Example Example Example Example Example Example 11 12 13 14 1516 Manufacturing Resin beads structure — Sheath- Sheath- Sheath- Sheath-Sheath- Sheath- conditions core core core core core core Cover- Poly-kinds — Resin A Resin A Resin A Resin A Resin A Resin A ing olefinMelting ° C. 125 125 125 125 125 125 layer resin point Anti- kinds — PEOPEO PEEA PEO PEO PEO static Blending % by 30 30 30 30 30 15 agent amountweight Ratio % by 5 5 5 5 5 15 weight Core Poly- kinds — Resin 2 Resin 3Resin 1 Resin 1 Resin 1 Resin 1 layer olefin Melting ° C. 142 155 142142 142 142 resin point Anti- kinds — — — — — — — static Blending % by 00 0 0 0 0 agent amount weight Ratio % by 95 95 95 95 95 85 weightMelting point difference ° C. 17 30 17 17 17 17 (core layer-coveringlayer) Blowing kinds — CO₂ CO₂ CO₂ CO₂ CO₂ CO₂ agent Internal MPa (G)2.60 2.15 2.65 2.85 3.40 2.80 pressure Foaming temperature ° C. 152.0161.0 145.5 145.0 144.5 145.5 Physical Apparent density kg/m³ 60 60 6040 32 60 properties of Average cell diameter μm 190 200 350 250 310 210foamed resin Shrinking ratio — 1.00 1.00 1.00 1.00 1.00 1.00 beadsCalorific value of a high J/g 15 19 16 16 15 14 temperature peakCalorific value as to all of J/g 75 88 79 79 79 67 the foamed resinbeads Manufacturing Molding pressure MPa (G) 0.32 0.40 0.32 0.26 0.240.28 conditions Physical Apparent density kg/m³ 67 67 67 44 36 67properties Fusion rate % 100 100 90 100 100 100 of molded Surface Skinsurface Ω 1.5 × 10¹² 2.6 × 10¹² 8.9 × 10¹² 9.9 × 10¹¹ 8.7 × 10¹¹ 8.7 ×10¹² foamed resistivity Cut surface Ω 5.0 × 10¹² 4.7 × 10¹² 9.1 × 10¹²2.8 × 10¹² 9.9 × 10¹¹ 1.2 × 10¹³ article 50% compressive stress kPa 540700 630 380 300 510 Example Example Example Example Example 17 18 19 2021 Manufacturing Resin beads structure Sheath- Sheath- Sheath- Sheath-Sheath- conditions core core core core core Cover- Poly- kinds Resin AResin A Amorphous Resin C Resin A ing olefin PP layer resin Melting 125125 (64)* 100 125 point Anti- kinds PEO PEO PEO PEO PEO static Blending15 15 30 30 30 agent amount Ratio 5 5 5 5 1 Core Poly- kinds Resin 1Resin 1 Resin 1 Resin 4 Resin 2 layer olefin Melting 142 142 142 142 142resin point Anti- kinds PEO — — — — static Blending 10 0 0 0 0 agentamount Ratio 95 95 95 95 99 Melting point difference 17 17 (78)** 62 17(core layer-covering layer) Blowing kinds air CO₂ CO₂ CO₂ CO₂ agentInternal 3.00 3.05 2.65 2.20 2.55 pressure Foaming temperature 147.5145.5 145.5 170.0 152.0 Physical Apparent density 66 18 60 60 60properties of Average cell diameter 110 400 150 200 190 foamed resinShrinking ratio 0.91 1.00 1.00 1.00 1.00 beads Calorific value of a high16 15 16 30 15 temperature peak Calorific value as to all of 70 80 75 9976 the foamed resin beads Manufacturing Molding pressure 0.28 0.22 0.280.50 0.34 conditions Physical Apparent density 67 20 67 67 67 propertiesFusion rate 100 100 100 100 80 of molded Surface Skin surface 3.6 × 10¹²6.2 × 10¹¹ 3.0 × 10¹² 1.0 × 10¹² 1.4 × 10¹³ foamed resistivity Cutsurface 6.6 × 10¹² 4.4 × 10¹² 4.9 × 10¹² 2.0 × 10¹² 8.9 × 10¹³ article50% compressive stress 580 180 640 720 555 *A value in parenthesislocated at an intersection of a column of the melting point and a row ofExample 19 is a softening point. **A value in parenthesis located at anintersection of a column of the melting point difference and a row ofExample 19 is the difference between the melting point of the core layerand the softening point of the covering layer.

TABLE 4 Comparative Comparative Comparative Comparative example 1example 2 example 3 example 4 Manufacturing Resin beads structure —Single- Single- Single- Sheath- conditions layer layer layer core Cover-Poly- kinds — — — — Resin 1 ing olefin Melting ° C. — — — 142 layerresin point Anti- kinds — — — — PEO static Blending % by — — — 15 agentamount weight Ratio % by — — — 5 weight Core Poly- kinds — Resin 2 Resin2 Resin 1 Resin 1 layer olefin Melting ° C. 142 142 142 142 resin pointAnti- kinds — PEEA PEEA PEO PEO static Blending % by 10 15 10 10 agentamount weight Ratio % by 100 100 100 95 weight Melting point difference° C. — — — 0 (core layer-covering layer) Blowing kinds — CO₂ CO₂ CO₂ CO₂agent Internal MPa (G) 2.10 1.90 2.15 2.65 pressure Foaming temperature° C. 152.0 152.0 145.5 145.5 Physical Apparent density kg/m³ 65 76 63 60properties of Average bubble diameter μm 400 550 240 250 foamed resinShrinking ratio — 0.92 0.79 0.96 1.00 beads Calorific value of a highJ/g 15 14 15 15 temperature peak Calorific value as to all J/g 63 69 6267 of the foamed resin beads Manufacturing Molding pressure MPa (G) 0.340.34 0.36 0.30 0.32 0.32 0.34 conditions Physical Apparent density kg/m³67 67 — 67 — 67 — properties Fusion rate % 50 30 — 70 — 60 — of moldedSurface Skin surface Ω 4.4 × 10¹⁴ 5.2 × 10¹² — 1.8 × 10¹³ — 2.5 × 10¹² —foamed resistivity Cut surface Ω 8.2 × 10¹⁴ 7.9 × 10¹² — 2.2 × 10¹³ —2.0 × 10¹² — article 50% compressive stress kPa 510 480 — 560 — 600 —Comparative Comparative Comparative example 5 example 6 example 7Manufacturing Resin beads structure Sheath- Sheath- Sheath- conditionscore core core Cover- Poly- kinds Resin A Resin A Resin A ing olefinMelting 125 125 125 layer resin point Anti- kinds PEO PEO — staticBlending 5 50 0 agent amount Ratio 5 5 5 Core Poly- kinds Resin 1 Resin1 Resin 1 layer olefin Melting 142 142 142 resin point Anti- kinds — —PEO static Blending 0 0 10 agent amount Ratio 95 95 95 Melting pointdifference 17 17 17 (core layer-covering layer) Blowing kinds CO₂ CO₂CO₂ agent Internal 2.65 2.65 2.25 pressure Foaming temperature 145.5145.5 145.5 Physical Apparent density 60 63 63 properties of Averagebubble diameter 150 150 250 foamed resin Shrinking ratio 1.00 0.95 0.95beads Calorific value of a high 16 16 15 temperature peak Calorificvalue as to all 80 79 71 of the foamed resin beads Manufacturing Moldingpressure 0.28 0.32 0.34 0.26 conditions Physical Apparent density 67 67— 67 properties Fusion rate 100 50 — 100 of molded Surface Skin surface8.7 × 10¹⁴ 1.0 × 10¹² — 1.8 × 10¹⁴ foamed resistivity Cut surface 5.4 ×10¹⁵ 1.5 × 10¹² — 1.6 × 10¹⁴ article 50% compressive stress 640 640 —590 Remark: The symbol “—” in a column of the physical properties ofmolded foamed article means not evaluated because the molded foamedarticle was significantly shrunk and a good foamed molded article wasnot given.

1. Foamed polyolefin resin beads obtained by foaming and expandingcomposite resin beads which include a core layer constituted by apolyolefin resin and a covering layer which covers the core layerconstituted by a polyolefin resin, wherein (a) the polyolefin resinconstituting the core layer is a crystalline polyolefin resin, (b) thepolyolefin resin constituting the covering layer is a crystallinepolyolefin resin which has a lower melting point (B) than a meltingpoint (A) of the polyolefin resin constituting the core layer, wherein atemperature difference [(A)−(B)] between the melting point (B) and themelting point (A) is more than 0° C. and 80° C. or less, or anoncrystalline polyolefin resin which has a softening point (C) lowerthan the melting point (A) of the polyolefin resin constituting the corelayer wherein a temperature difference [(A)−(C)] between the softeningpoint (C) and the melting point (A) is more than 0° C. and 100° C. orless, and 10% by weight or more and less than 50% by weight of polymertype antistatic agent is contained in the covering layer.
 2. The foamedpolyolefin resin beads according to claim 1, wherein the polyolefinresin constituting the covering layer is a crystalline polyolefin resinwhich has a lower melting point (B) than a melting point (A) of thepolyolefin resin constituting the core layer, wherein a temperaturedifference [(A)−(B)] between the melting point (A) and the melting point(B) is in the range of 1 to 80° C., or a noncrystalline polyolefin resinwhich has a lower softening point (C) than a melting point (A) of thepolyolefin resin constituting the core layer, wherein a temperaturedifference [(A)−(C)] between the melting point (A) and the softeningpoint (C) is in the range of 1 to 100° C.
 3. The foamed polyolefin resinbeads according to claim 1, wherein the polyolefin resin constitutingthe covering layer is a crystalline polyolefin resin which has a lowermelting point (B) than a melting point (A) of the polyolefin resinconstituting the core layer, wherein a temperature difference [(A)−(B)]between the melting point (A) and the melting point (B) is in the rangeof 5 to 60° C., or a noncrystalline polyolefin resin which has a lowersoftening point (C) than a melting point (A) of the polyolefin resinconstituting the core layer, wherein a temperature difference [(A)−(C)]between the melting point (A) and the softening point (C) is in therange of 5 to 60° C.
 4. The foamed polyolefin resin beads according toclaim 1, wherein the core layer does not substantially contain thepolymer type antistatic agent.
 5. The foamed polyolefin resin beadsaccording to claim 1, wherein 5 to 15% by weight of polymer typeantistatic agent is contained in the core layer.
 6. The foamedpolyolefin resin beads according to claim 1, wherein the crystallinepolyolefin resin constituting the core layer is a polypropylene resin.7. The foamed polyolefin resin beads according to claim 6, wherein thepolyolefin resin constituting the covering layer is the polypropyleneresin.
 8. The foamed polyolefin resin beads according to claim 1,wherein a weight ratio of the core layer and the covering layer is inthe range of 99.5:0.5 to 80:20.
 9. The foamed polyolefin resin beadsaccording to claim 1, wherein the weight ratio of the core layer and thecovering layer is in the range of 98:2 to 80:20.
 10. The foamedpolyolefin resin beads according to claim 1, wherein the weight ratio ofthe core layer and the covering layer is in the range of 96:4 to 90:10.11. The foamed polyolefin resin beads according to claim 1, wherein thepolyolefin resin constituting the covering layer is a polyolefin resinpolymerized with a metallocene polymerization catalyst.
 12. The foamedpolyolefin resin beads according to claim 1, wherein a ratio (X/Y) of anapparent density (X) of foamed beads after pressurizing with acompressed air under conditions of 30° C. and 0.2 MPa (G) for 24 hoursand leaving under an ordinary pressure at 23° C. for 24 hours to anapparent density (Y) of foamed beads before the pressurization is in therange of 0.8 to 1.0.
 13. The foamed polyolefin resin beads according toclaim 1, wherein the covering layer of the foamed beads is substantiallysolid.
 14. A molded foamed article of polyolefin resin produced bymolding the foamed polyolefin resin beads according to claim 1 in amolding cavity, wherein the molded foamed article has the surfaceresistivity of less than 1′ 1014 W.